Log repeat stream closing on Debug level

internal/multiplex/session.go

@@ -20,6 +20,7 @@ const (
 
 var ErrBrokenSession = errors.New("broken session")
 var errRepeatSessionClosing = errors.New("trying to close a closed session")
+var errRepeatStreamClosing = errors.New("trying to close a closed stream")
 
 type switchboardStrategy int
 
@@ -145,7 +146,7 @@ func (sesh *Session) Accept() (net.Conn, error) {
 
 func (sesh *Session) closeStream(s *Stream, active bool) error {
 	if atomic.SwapUint32(&s.closed, 1) == 1 {
-		return fmt.Errorf("stream %v is already closed", s.id)
+		return fmt.Errorf("closing stream %v: %w", s.id, errRepeatStreamClosing)
 	}
 	_ = s.recvBuf.Close() // both datagramBuffer and streamBuffer won't return err on Close()
 

internal/multiplex/stream.go

@@ -62,7 +62,12 @@ func (s *Stream) isClosed() bool { return atomic.LoadUint32(&s.closed) == 1 }
 func (s *Stream) writeFrame(frame Frame) error {
 	toBeClosed, err := s.recvBuf.Write(frame)
 	if toBeClosed {
-		return s.passiveClose()
+		err = s.passiveClose()
+		if errors.Is(err, errRepeatStreamClosing) {
+			log.Debug(err)
+			return nil
+		}
+		return err
 	}
 	return err
 }